Hess's Law Calculator
Hess's law lets you find the enthalpy change of a reaction that is hard to measure directly by combining the enthalpies of reactions you do know. Because enthalpy is a state function, the total change depends only on the start and end states, so the enthalpies of a sequence of steps simply add. You reverse a step by using a negative multiplier and scale a step by any factor. This calculator sums up to three step enthalpies, each times its multiplier, to give the overall reaction enthalpy. Enter each step's enthalpy and the multiplier that makes the steps add to your target reaction.
Hess's law formula
delta H overall = m1 * H1 + m2 * H2 + m3 * H3
use a negative multiplier to reverse a step
use a fractional multiplier to scale a step
set a multiplier to 0 to ignore an unused step
Each step contributes its enthalpy times its multiplier. Reversing flips the sign; scaling multiplies the magnitude. The overall value is the sum of the three contributions.
Hess's law context
- Enthalpy is a state function, so step enthalpies add to give the overall change.
- Reversing a reaction reverses the sign of its enthalpy change.
- Scaling a reaction by a factor multiplies its enthalpy by the same factor.
- The method works for any path connecting the same reactants and products.
- It is the basis for calculating reaction enthalpies from formation enthalpies.
Hess's law: frequently asked questions
What is Hess's law?
Hess's law states that the total enthalpy change of a reaction is the same whether it occurs in one step or several steps. Because enthalpy is a state function, you can add the enthalpy changes of a series of steps to get the overall enthalpy change.
How do I use Hess's law?
Arrange known reactions so that they add up to the target reaction, reversing or scaling them as needed. Reversing a reaction changes the sign of its enthalpy, and scaling it by a factor multiplies its enthalpy by the same factor. Sum the adjusted enthalpies.
What happens when I reverse a reaction?
Reversing a reaction reverses its direction, so its enthalpy change changes sign. If a forward step releases 100 kJ (delta H = -100 kJ), the reverse step absorbs 100 kJ (delta H = +100 kJ). Enter a multiplier of -1 to reverse a step in this calculator.
Why can enthalpy changes be added?
Enthalpy is a state function, meaning it depends only on the initial and final states, not the path taken. So the sum of enthalpy changes along any path between the same start and end points is identical, which is the basis of Hess's law.
Can multipliers be fractions?
Yes. If a step must be scaled to half to match the target reaction, use a multiplier of 0.5. Fractional multipliers are common when balancing the target equation requires non-integer coefficients for the intermediate steps.
Official sources
- NIST Chemistry WebBook: thermochemical and enthalpy reference data.
- IUPAC Gold Book: enthalpy as a state function and Hess's law context.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.